1. Field of the Invention
The present invention concerns a method of manufacturing a composition for use in injection molding powder metallurgy, which composition enables the production of products of excellent dimensional accuracy.
2. Description of the Prior Art
The demand for products of complicated shapes has increased in recent years with an increase in demand for mechanical parts. Injection molding powder metallurgy has been employed to obtain products of complicated shapes using a composition comprising a mixture of a metal powder and a, i.e. binder using an extrusion molding machine having substantially the same structure as that of an injection molding machine used for plastic molding, removing binders from the molded products and then sintering with heat. In injection molding powder metallurgy, since a great amount of binder has to be used in forming a metal powder composition, the molded product after the removal of the binder is porous and the density of the sintering product can not be increased. In view of the above, it has been attempted to obtain a high density sintering product by using a highly sinterable fine powder or a powder of spherical granular shape which packs so well as to reduce the amount of binder required. That is, it has been proposed to produce an injection molding composition by using a fine water-atomized powder or a spherical gas-atomized powder classified to not greater than 50 .mu.m as the starting powder for injection molding, and mixing and kneading such a powder with a binder.
However, even though the product obtained by water-atomization is a fine powder is characterized by irregular granular shape although it is a fine powder. Accordingly, a binder has to be added to the extent of at least 45% by volume of the composition, which leads to not only a deterioration in the strength of the molded product after the binder-removing treatment, but also a reduction in handlability, and, in addition, shape defects such as warps or voids when sintered. On the other hand, a powder obtained by gas atomization has an average grain size of more than 50 .mu.m and the injection moldability of the composition is poor since the grain size is quite coarse. In addition, the molded product has poor sinterability and the sintering density can not be increased as it is. Accordingly, it is necessary to sieve the average grain size to not greater than 50 .mu.m in a case of a gas-atomized powder and, thus, only a portion of the powder obtained by gas atomization can be used, leading to the disadvantage of increased cost.